senna

I went to see to buy a gs but it had a weird problem.

the owner had the tank filled up (3/4) right before i was there.
the engine started oke and kept on running oke for a while . when we shut it down there started to drip fuel from an overflow tube on the side .we than started the motor again but it went a bit difficult. we got it runnig again , and it stayt on whitout a problem . but when we shut it down the dripping fuel was there again .

now the obvious question is ,what is causing this? could it be enough to do some maintenance on the carb?

thx in advance.
PS sorry for speaking in English ,but I am from Belgium so I don't speak German.